Semi-conductor devices or modules are known in which a multiplicity of semi-conductor chips are solder-bonded on a ceramic substrate or carrier, at chip sites thereon. In such devices, there is a need for the ability to replace a chip at a given chip site one or more times. To accomplish this, the old chip is removed, the chip site is dressed to remove any excess solder, the chip site is fluxed and a new chip with solder balls thereon is attached by solder reflow. The present invention relates to the step of dressing the chip site.
In conducting a dressing operation, it is desirable that the operation be performed as fast and as cheaply as possible, that it not affect any wires or other chips mounted on the substrate, and that it does not cause any solder bridging between adjacent chip site pads. In addition, in order to prevent the accumulated build-up of solder during subsequent rework operations at the same site, the dressing operation should provide control over the volume of solder that remains after dressing. One problem with volume control is that due to the manner of making the ceramic substrates, the surface of the substrate at chip site is uneven. Additionally, it is also necessary to control the shape of whatever solder remains at the different pads after dressing, in order to facilitate subsequent chip placement and rejoining.
Within the prior art, there are many different methods and apparatus used for chip site dressing. In general, the different ways can be classed as either a thermal method or a mechanical method. In thermal methods, the solder is generally melted and removed either by blowing, sucking, or wetting a wick or simulated chip so that when the wick or chip is removed, it carries with it any excess solder. In mechanical methods, the solder remaining after removal is either flattened or it is shaved off by some form of a cutting tool or chisel. The problem with these mechanical methods is they generally fail to take into account surface unevenness at the chip site and solder columns at high spots will be flattened to a greater extent or cut off more than those at low spots on the chip site, so that the amount or volume of material might differ. In general, such prior art methods fail to achieve all of the desirable results of a successful dressing operation, as discussed above.